Rust never sleeps—Observations of electron
hopping in iron oxide hold consequences
for environment and energy
7 September 2012, by Lynn Yarris
transferred to an iron oxide particle.
"We believe this work is the starting point for a new
area of time-resolved geochemistry that seeks to
understand chemical reaction mechanisms by
making various kinds of movies that depict in real
time how atoms and electrons move during
reactions," says Benjamin Gilbert, a geochemist
with Berkeley Lab's Earth Sciences Division and a
co-founder of the Berkeley Nanogeoscience Center
who led this research. "Using ultrafast pump-probe
X-ray spectroscopy, we were able to measure the
rates at which electrons are transported through
spontaneous iron-to-iron hops in redox-active iron
oxides. Our results showed that the rates depend
on the structure of the iron oxide and confirmed that
certain aspects of the current model of electron
hopping in iron oxides are correct."
Iron oxide (rust) is a poor electrical conductor, but
electrons in iron oxide can use thermal energy to hop
from one iron atom to another. A Berkeley Lab
experiment has now revealed exactly what happens to
electrons after being transferred to an iron oxide particle. Gilbert is the corresponding author of a paper in the
(Image courtesy of Benjamin Gilbert, Berkeley Lab)
journal Science that describes this work. The paper
is titled "Electron small polarons and their mobility
in iron (oxyhydr)oxide nanoparticles." Co-authoring
the paper were Jordan Katz, Xiaoyi Zhang, Klaus
(Phys.org)—Rust—iron oxide—is a poor conductor ofAttenkofer, Karena Chapman, Cathrine Frandsen,
electricity, which is why an electronic device with a Piotr Zarzycki, Kevin Rosso, Roger Falcone and
rusted battery usually won't work. Despite this poor Glenn Waychunas.
conductivity, an electron transferred to a particle of
rust will use thermal energy to continually move or At the macroscale, rocks and mineral don't appear
"hop" from one atom of iron to the next. Electron
to be very reactive – consider the millions of years it
mobility in iron oxide can hold huge significance for takes for mountains to react with water. At the
a broad range of environment- and energy-related nanoscale, however, many common minerals are
reactions, including reactions pertaining to uranium able to undergo redox reactions – exchange one or
in groundwater and reactions pertaining to low-cost more electrons – with other molecules in their
solar energy devices. Predicting the impact of
environment, impacting soil and water, seawater as
electron-hopping on iron oxide reactions has been well as fresh. Among the most critical of these
problematic in the past, but now, for the first time, a redox reactions is the formation or transformation of
multi-institutional team of researchers, led by
iron oxide and oxyhydroxide minerals by chargescientists at the U.S. Department of Energy
transfer processes that cycle iron between its two
(DOE)'s Lawrence Berkeley National Laboratory
common oxidation states iron(III) and iron(II).
(Berkeley Lab) have directly observed what
happens to electrons after they have been
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"Because iron(II) is substantially more soluble than
iron(III), reductive transformations of iron(III) oxide
and oxyhydroxide minerals can dramatically affect
the chemistry and mineralogy of soils and surface,"
Gilbert says. "In the case of iron(III) oxide, the
reduction to iron(II) can cause mineral dissolution
on a very fast timescale that changes the
mineralogy and water flow pathways. There can
also be a mobilization of iron into solution that can
provide an important source of bioavailable iron for
living organisms."
because mineral redox reactions are complex and
involve multiple steps that take place within a few
billionths of a second. Until recently these reactions
could not be observed, but things changed with the
advent of synchrotron radiation facilities and
ultrafast X-ray spectroscopy.
"Much like a sports photographer must use a
camera with a very fast shutter speed to capture an
athlete in motion without blurring, to be able to
watch electrons moving, we needed to use a
exceedingly short and very bright (powerful) pulse
of X-rays," says Jordan Katz, the lead author on the
Science paper who is now with Denison University.
"For this study, the X-rays were produced at
Argonne National Laboratory's Advanced Photon
Source."
In addition to short bright pulses of X-rays, Katz
said he and his co-authors also had to design an
experimental system in which they could turn on
desired reactions with an ultrafast switch.
"The only way to do that on the necessary
timescale is with light, in this case an ultrafast
laser," Katz says. "What we needed was a system
in which the electron we wanted to study could be
immediately injected into the iron oxide in response
to absorption of light. This allowed us to effectively
synchronize the transfer of many electrons into the
iron oxide particles so that we could monitor their
aggregate behavior."
In an iron oxide lattice, an electron sits on an iron(II) atom
(yellow center). Its negativity attracts the more positive
iron(III) atoms (red circles) and repels the more negative
oxygen atoms (white circles), creating a physical
distortion in the lattice from which the electron escapes
only with the help of heat energy.
With their time-resolved pump-probe spectroscopy
system in combination with ab initio calculations
performed by co-author Kevin Rosso of the Pacific
Northwest National Laboratory, Gilbert, Katz and
their colleagues determined that the rates at which
electrons hop from one iron atom to the next in an
iron oxide varies from a single hop per nanosecond
Gilbert also noted that many organic and inorganic to five hops per nanosecond, depending on the
structure of the iron oxide. Their observations were
environmental contaminants can exchange
consistent with the established model for describing
electrons with iron oxide phases. Whether it is
electron behavior in materials such as iron oxides.
iron(III) or iron(II) oxide is an important factor for
In this model, electrons introduced into an iron
degrading or sequestering a given contaminant.
Furthermore, certain bacteria can transfer electrons oxide couple with phonons (vibrations of the atoms
in a crystal lattice) to distort the lattice structure and
to iron oxides as part of their metabolism, linking
create small energy wells or divots known as
the iron redox reaction to the carbon cycle. The
polarons.
mechanisms that direct these critical
biogeochemical outcomes have remained unclear
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"These electron small polarons effectively form a
localized lower-valence metal site, and conduction
occurs through thermally-activated electron
hopping from one metal site to the next," Gilbert
says. "By measuring the electron hopping rates we
were able to experimentally demonstrate that
iron(II) detachment from the crystal is rate-limiting
in the overall dissolution reaction. We were also
able to show that electron hopping in the iron
oxides is not a bottleneck for the growth of
microbes that use these mineral as electron
acceptors. The protein-to-mineral electron transfer
rate is slower."
Katz is excited about the application of these
results to finding ways to use iron oxide for solar
energy collection and conversion.
"Iron oxide is a semiconductor that is abundant,
stable and environmentally friendly, and its
properties are optimal for absorption of sunlight," he
says. "To use iron oxide for solar energy collection
and conversion, however, it is critical to understand
how electrons are transferred within the material,
which when used in a conventional design is not
highly conductive. Experiments such as this will
help us to design new systems with novel
nanostructured architectures that promote desired
redox reactions, and suppress deleterious reactions
in order to increase the efficiency of our device."
Adds Gilbert, "Also important is the demonstration
that very fast, geochemical reaction steps such as
electron hopping can be measured using ultrafast
pump-probe methods."
More information: Jordan E. Katz, Xiaoyi Zhang,
Klaus Attenkofer, Karena W. Chapman, Cathrine
Frandsen, Piotr Zarzycki, Kevin M. Rosso, Roger
W. Falcone, Glenn A. Waychunas, Benjamin
Gilbert. Electron Small Polarons and Their Mobility
in Iron (Oxyhydr)oxide Nanoparticles, Sept. 7,
2012, Science, doi: 10.1126/science.1223598.
Provided by Lawrence Berkeley National
Laboratory
APA citation: Rust never sleeps—Observations of electron hopping in iron oxide hold consequences for
environment and energy (2012, September 7) retrieved 15 June 2017 from
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https://phys.org/news/2012-09-rust-sleepsobservations-electron-iron-oxide.html
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